Dynamic mixers are known which comprise two elements or mixing parts which are rotatable relative to each other about a predetermined axis and between which is defined a flow path extending between an inlet for materials to be mixed and an outlet. In such known mixers, the flow path is defined between surfaces of the mixing parts, each of which surfaces has cavities formed within it. Cavities formed in one surface are offset in the axial direction relative to cavities in the other surface, and cavities in one surface overlap in the axial direction with cavities in the other surface. As a result, material moving between the surfaces is transferred between overlapping cavities. Thus, in use, material to be mixed is moved between the mixing parts and traces a path through cavities located alternately on each of the two surfaces. Such mixers incorporating cavities are generally referred to as “cavity transfer mixers”.
Known cavity transfer mixers may have either a cylindrical geometry, that is an inner mixing part having a generally cylindrical outer surface which typically forms a rotor of the device and an outer mixing part having a generally cylindrical inner surface which typically forms a stator of the device, or, more recently, a stepped conical geometry, that is an inner mixing (rotor) part having a generally conical outer surface and an outer mixing (stator) part having a generally conical inner surface. In both cases, rows of cavities are formed in the two facing outer and inner surfaces, the rows of cavities overlapping in the axial direction such that material to be mixed generally passes from a cavity in one row of one surface into a cavity in an adjacent row of the other surface.
Stepped conical geometry cavity transfer mixers, for example as described in WO02/38263A1, are preferred over cylindrical cavity transfer mixers for a number of reasons, including that with a cylindrical mixer it is often necessary to manufacture the outer stator in splittable form so as to enable the formation of rows of cavities in its inner surface, whereas the complementary stepped conical geometry of each of the stator and rotor in a stepped conical geometry cavity transfer mixer means that each part can be manufactured whole with the cavities easily formable in both surfaces; the lack of an open annular space between the stator and rotor with a stepped conical geometry mixer, unlike a cylindrical mixer, which reduces the likelihood of material passing straight through said space effectively bypassing the cavities; the reduced likelihood of asymmetrical transfers with a stepped conical geometry mixer, unlike a cylindrical mixer, which transfers may cause axial back flow or front flow that can generate stagnation patterns with resultant accumulation of material in the cavities; and the lack of self-pumping and/or self-cleaning capabilities with cylindrical cavity transfer mixers, to name a few. However, by far the main reason for choosing a stepped conical geometry cavity transfer mixer, particularly that described in WO02/38263A1, in preference to a cylindrical cavity transfer mixer lies is because of the excellent distributive and dispersive mixing that can be achieved.
The aim of distributive mixing of a material is to improve the spatial distribution and uniformity of its components, with any inherent cohesive resistance in the material playing an insignificant role. Distributive mixing is also sometimes referred to as “simple mixing” or “extensive mixing”. With dispersive mixing however, inherent cohesive resistance in the material(s) being mixed has to be overcome in order to achieve finer levels of dispersion. Dispersive mixing is also sometimes referred to as “intensive mixing” and is often more difficult to achieve than distributive mixing; this was true at least until the advent of the stepped conical geometry transfer, especially that described in WO02/38263A1, which as noted above enables achievement of excellent distributive and dispersive mixing.
However, despite the clear advantages of using a stepped conical geometry cavity transfer mixer over a cylindrical geometry cavity transfer mixer, it would be desirable to improve upon the excellent distributive and dispersive mixing achievable with a known stepped conical geometry mixer, for example by reducing the magnitude of the pressure drop that may occur on material transfer from the inlet to the outlet, by optimising the extensional and/or shear stresses applied to a material being mixed by suitable control thereof, and by the ability to provide uniform shear and extensional stresses.
It is therefore an object of the present invention to provide a cavity transfer mixer that is improved as compared to known cavity transfer mixers, particular as compared to stepped conical geometry mixers, especially (but not essentially) in relation to any of the desired aspects described in the preceding paragraph.